Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials. One of physical properties required for many optical materials is a high refractive index. With respect to optical materials having a high refractive index, many episulfide compounds by which optical materials having a refractive index of 1.7 or more can be obtained have been found (see Patent Documents 1, 2 and 3). Since most of compositions comprising these compounds are cured by means of thermal curing, intended use thereof is significantly limited, and a composition which can be photocured has been strongly desired.
Photo-curing of episulfide compounds is described in Patent Documents 4, 5, 6, 7 and 8. These documents disclose a radical generator, an acid generator, a base generator, etc. as initiators, but since base catalysts have the highest activity against polymerization of episulfide compounds, it is considered that a base generator is most desirably used. However, research on photobase generators has only a short history, and no photobase generator having polymerization activity enough for practical use has been found. Research on photobase generators is described in Non-Patent Documents 1 and 2.
Major problems of the photobase generators are that light absorption of episulfide compounds at up to near 300 nm almost overlaps the light absorption region of the photobase generators, resulting in low efficiency of generation of base by means of photolysis, and that since the basicity of base generated by means of photolysis is weak, polymerization and curing of episulfide compounds is slow.
Patent Document 9 proposes a photobase generator which improves the above-described problems. However, a composition obtained by adding the photobase generator to an episulfide compound has a short pot life, the viscosity of the composition is significantly increased within several hours to a dozen or so hours, and the composition is cured within several days. Thus, there is a problem that it is difficult to handle the composition because of short pot life (working life) thereof.